As is well known, television receivers are susceptible to signal interference from various noise sources. This interference may arise from many different sources which operate to interfere with the television signal or with the processing of the television signal by the television receiver. Typical sources of such interference are automotive ignition systems, household motors and various other forms of interference as well. The term impulse noise interference is commonly used to describe such events and can cause disruption to the automatic gain control (AGC) circuits, the synchronizing circuits, as well as the video and chroma signals. As is well known, if the impulse noise is present in the video detector output signal, that noise can proceed through the video processing path and result in a noise image being developed on the screen of the picture tube. The impulse noise will also be supplied to the sync processing path and cause the sync separator to generate unwanted output signals. Because the horizontal sync from the sync separator is usually supplied to an AGC system in the receiver, the latter system may be disrupted by the noise-induced output of the sync separator. As indicated, these problems are well known in the prior art.
AGC and sync circuits are limited-bandwidth systems and filtering has been used to render these circuits relatively immune to impulse noise. Video and chroma circuits cannot employ the filtering techniques that are used with the sync and AGC circuits because the impulse noise signals share the same frequency spectrum with the video and chroma signals. Accordingly, non-linear signal processing of some sort is often applied to such receivers, but this processing often is not very effective.
In U.S. Pat. No. 4,377,823 issued on Mar. 22, 1983 and entitled "Noise Processing System for a Television Receiver" Mycynek describes a television receiver with a video detector of envelope type which develops only black-going impulse noise, which black-going impulse noise is detected and inverted in the sync processing path. Mycynek further describes the black-going impulse noise in the video processing path being detected and replaced by a constant video level, preferably a 30 IRE gray level.
In U.S. Pat. No. 4,514,763 issued Apr. 30, 1985 and entitled "Sound Signal and Impulse Noise Detector for Television Receivers" Rindal describes a television receiver using a phase-lock loop for detecting the audio information from the television signal and for providing a control signal which, when applied to compensation circuitry, reduces impulse noise effects in the video information. Rindal refers to the problem of sorting impulse noise response from desired video signal and avoids this problem by detecting impulse noise, not as it accompanies video signal, but rather as it modulates the amplitude of the frequency-modulated sound carrier.
Impulse noise is a problem in other types of systems, such as AM radio, where the logarithmic characteristic of one's hearing response helps diminish the intrusiveness of impulse noise during listening. In regard to noise cancellation techniques in general, reference is made to the following U.S. patents which pertain to the general field of methods and techniques for cancelling impulse noise. In U.S. Pat. No. 4,272,846 issued Jun. 9, 1981 and entitled "Method for Cancelling Impulse Noise" Muratani et alii describe a method for cancelling impulse noise in a system where a band-limited baseband signal is transmitted through a channel that has a wider band than that of the baseband signal. In U.S. Pat. No. 4,810,101 issued Mar. 7, 1989 and entitled "Noise Detection by Sampling Digital Baseband Signal at Eye Openings" Kage et alii describe a noise detection circuit for a digital radio receiver wherein a signal is sampled at a particular time interval when a large noise pulse is generated. In U.S. Pat. No. 4,622,520 issued Nov. 11, 1986 and entitled "FM Demodulator With Impulse Noise Elimination Circuit" Kuroda describes apparatus for demodulating an FM signal in which apparatus impulse noise is eliminated by a noise elimination and detection circuit which appears before the filters. Other patents describe noise pulse suppressing systems for mobile communications radio receivers, such as U.S. Pat. No. 4,311,963 entitled "Noise Pulse Suppressing System" issued Jan. 19, 1982 to Watanabe et alii. U.S. Pat. Nos. 4,272,846 and 4,311,963 are of particular interest in that both disclose the general concept of detecting impulse noise in a signal, subsequently delaying that signal, and then responsive to the detection of the impulse noise cancelling the impulse noise in the delayed signal. U.S. Pat. No. 4,311,963 is also of particular interest for its disclosure of prior art use of track-and-hold circuitry in impulse noise cancellation schemes
While there is ample evidence of a general awareness of noise cancellation and detection methods on the part of those skilled in the art, modern television receivers operate in various modes which introduce new problems of impulse noise detection. High-performance television receivers often employ synchronous picture (pix) intermediate frequency (IF) demodulators. Synchronous demodulation may be done in two phases: an in-phase synchronous demodulation that detects the composite video signal and the accompanying modulated sound carrier, and a quadrature-phase synchronous demodulation that detects the chrominance signal and modulated sound carrier without much accompanying luminance information. The only baseband components in the quadrature-phase synchronous demodulator response are differentiated transients of sync pulses and luma.
Unlike envelope or peak detectors which invariably detect the impulse noise as black-going in a negatively modulated video carrier such as that used in the NTSC and PAL television broadcast standards, synchronous detectors demodulate the asynchronous impulse noise as alternately black-going and white-going noise. White-going impulse noise is particularly objectionable since it tends to bloom the picture tube. The amplitude-modulated video carrier is vestigial sideband, so the pix IF amplifier chain filtering is centered about 2 MHz away from the video carrier frequency. Ringing of this filtering by impulse noise generates a random-phase damped sinusoid of about 2 MHz frequency, usually of large amplitude, in the in-phase synchronous demodulator response. If a quadrature-phase synchronous demodulator is used, a random-phase damped sinusoid of comparable frequency and amplitude is also generated in the quadrature-phase synchronous demodulator response.
In U.S. Pat. No. 4,524,389 issued Jun. 18, 1985 and entitled "Synchronous Video Detector Using Phase-Locked Loop" Isobe et alii describe a television receiver having just an in-phase synchronous demodulator. Black-going impulse noise in the output signal of this video detector is detected by a black noise detector and is thereafter cancelled to gray. The output signal of the black noise detector is supplied to a pulse-stretcher. The pulse stretcher output signal is used to control the cancellation to gray of white noise following the detected black noise. The Isobe et alii procedure has shortcomings. Likely as not, the initial signal swing of the synchronously detected impulse noise with significant energy will be white-going, rather than black-going. Each such a white-going initial swing undesirably causes an intense white spot in the picture. Collectively, these white spots are sometimes called "salt" noise in contradistinction to "pepper" noise, a term used to refer collectively to the black spots in the picture caused by inversion of impulse noises to black in a television receiver with a video detector of the envelope detector type. These white-going spikes in the video detector output signal also disrupt the chroma channel.
The inventor knows of previous techniques for suppressing white-going impulse noise in which the white-going impulse noise in the video detector output signal is sensed and subsequently replaced with black (or a prescribed gray) level to generate a modification of video detector response. The setting of the video noise inversion threshold in such systems is extremely critical. The depth of video modulation can vary considerably from one source to another; so, if the threshold for impulse noise detection is set too close to the white level, false tripping on high white level modulation will frequently occur. If there is a high chroma, due to standing waves or other antenna problems, the noise inverter will falsely trigger on the chroma signal. If the threshold level is too high, too much white-going impulse noise will get through and bloom the picture tube. The detected in-phase video signal generally changes to white before it is detected as white-going impulse noise, so the damage or interference to the picture is already apparent when action is instituted to suppress the white-going impulse noise in these previous techniques. Although the duty factor of the white-going impulse noise is reduced, the interference is still seen by a person viewing the televised picture.
As practiced in the prior art, the very act of noise inversion creates a high-slew-rate signal which propagates through the video and chroma channels of the television system or receiver. A black or gray streak is inserted in the video signal by noise inversion circuitry that responds to impulse noise to replace the noise with a prescribed video level, and this streak is readily evident on the screen when impulse noise occurs over an extended time. The chroma channel is shock excited by the large-amplitude, fast-rising noise inversion pulse; and the consequent ringing of the filters in the chroma channel causes chroma "twinkle". Chroma "twinkle" comprises color changes of short duration at the points in the television picture where impulse noise intermittently occurs. The color changes at each of which points reminds some viewers of the light emitted by a star, which is the reason the word "twinkle" is associated with this phenomenon.
The problem of the detected in-phase video signal changing to white before impulse noise is detected, so the damage or interference to the picture is already apparent when cancellation of impulse noise proceeds, is avoided in the invention by detecting the impulse noise as it occurs in a video detector output signal and effecting noise cancellation in a delayed response to that or another video detector output signal. The television picture is subsequently derived from the delayed video signal after the noise cancellation. If the impulse noise is detected in the video signal supplied by an in-phase synchronous demodulator and is detected in only one sense, black-going or white-going, it is preferable to detect white-going impulse noise. The video detector output signal can be delayed a shorter time before effecting noise cancellation, while still avoiding blooming on white-going noise, thereby reducing hardware cost.
The problem of chroma "twinkle" is addressed in the invention by using track-and-hold circuitry to effect noise cancellation in the delayed video detector output signal, thereby to avoid introducing a large-amplitude, fast-rising noise inversion pulse into that delayed signal, as would shock excite the chroma channel using that delayed signal for input signal. Effecting noise cancellation in a delayed response to video detector output signal in order to avoid white-spotting during the initial portions of impulse noise removes another source of shock excitation of the chroma channel using that delayed signal for input signal.